drivers: spi_context: fix null pointer dereferencing

commit 54907c70
("drivers: spi: spi_context: improve support of multiple cs gpios")
added function to initialize all CS GPIOs defined in devicetree.
This function, spi_context_cs_configure_all, is intended to be
called during SPI driver initialization (POST_KERNEL init level).
It is also obvious that a SPI driver was not used at that time,
and no bus configuration (struct spi_config) is assigned to
SPI bus (spi_context.config).

The spi_context_cs_configure_all function has a homeopathic
ASSERT to validate CS levels, which causes a null pointer
dereferencing by ctx->config->operation if asserts are
enabled.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>